Variable gain circuit device



Dec. 20, 1932. MNTRWGHAM I 1,891,297

VARIABLE GAIN G IRCUIT DEVICE Filed June 25. 1929 5 Sheets-Sheet 1 4 17 l|l Filter INVENTOR 715%. Whiz in km ATTORN EY Dec. 20, 1932.

Out at Volts Irzput'V its 1.4;" Scalexi W. T. WINTRINGHAM VARIABLE GAIN CIRCUIT DEVICE Filed June 25, 1929 Faraokl Scale X 79 7/ I I Cubic I I scale/Y 700 3 Sheets-Sheet 2 Eeqaz'rezi Gridflz'as S. Input Wits Cato/71 65135 Bah/202d Zhmsmzssww Balanced Zhmsmrlysim Balanced at 0 11 Praduced a Ji-g'ule Emmy INVENTOR ATTORNEY 2- w. -T. WINTRINGHAM 1,891,297

VARIABLE GAIN CIRCUIT DEVICE Filed June 25, 1929 5 Sheets-Sheet 3 (Required Carve M INVENTOR -e Wis Wfmlwa'lrgglzam BY 2, 9m

TTORNEY Patented Dec. 20, 1932 UNITED. STATES PATENTI'QFFIZCEI WILLIAM T. WINTRINGHAM, OF BROOKLYN, NEW YORK, ASSIGN OR TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A CGRPORATION OF NEW YORK VARIABLE GAIN Application fil d June 25,

' work is provided across the network terminals of the hybrid coil. By varying the grid potential on the vacuum tube in the control circuit, the plate circuit impedance of this tube is changed and hence causes a change in the hybrid balance in the hybrid coil circuit. This in turn will result in variations in the loss introduced by the hybrid coil in the transmission circuit and such variations in loss may be controlled by the potentials applied to the control circuit.

A specific illustration of a situation wherein the variable gain device of this invention might be utilized to advantage would be in systems wherein it might be desired to limit the power or voltages to be transmitted over a circuit therein. One of the methods of limiting the power or voltages which may be transmitted over a circuit comprises dividing the main transmission path into two branch circuits and connecting the output circuits of these branch circuits in opposition to another section of the main transmission circuit. Through one of these branch paths, provided with an amplifier and a delay circuit. the signals pass in the ordinary way. In the other branch path there may be included the variable gain device of this invention. The control circuit of the device will be effected by the incoming signal. energy and the control tube and hybrid coil combination will be so adjusted that for values of signals below the desired cut-off point, little or no power will be transmitted through the device, the eby leaving the first mentioned branch path be normally the dominant one. The control tube and hybrid coil arrangement in the second branch path will also be so adjusted that when the signaling power ap- CIBCUIT DEVICE 1929. Serial No. 873,679.

iting value of the overall transmission through the circuit. ther features and applications of the arrangements of the invention will appear more fully from the detailed description thereof hereinafter given.

The inventionmay be more fully understood from the following description together with the accompanying drawings in the Figures 1, 2, 2(a) 3, a, 5, 6 and 7 of whichthe in vention is illustrated. In Fig. lis a circuit diagram embodying the invention. Fig. 2 is a circuit diagram showing the arrangements of the invention as applied to an arrangement for voltage limiting purposes. Fig. 2 a) is a schematic diagram showing the double bridge arrangement of Fig. 2. Figs. 3, 4, 5, 6 and 7 are graphical illustrations otcertain of the operating principles of the invention.

In Fig. 1 is shown a portion of a transmis-. sion circuit comprising aninput section L and an output section L These two line sections are interconnected by the hybrid coil 1. Connected by means of transformer 3 to the line side of the hybrid coil is the output or plate circuit of a three-element vacuum tube 2. This tube is connected to a control circuit L Connected by means of transformer 4 to the network side of the hybrid coil is a resistance element '5.' The voltage E applied to the control circuit L will change the bias on the grid of tube 2 and hence vary the current in the plate circuit of tube 2 and thus vary the impedance of the plate circuit of the tube. This, in turn,

will vary the hybrid balance of'the hybrid coil arrangement 1 and thus change the loss between the drop terminals thereof, or, in other words, between L and L Accordingly, it will be possible to vary the gain in the circuit comprising line sections L and L If the hybrid coil is balanced at E =E the hybrid balance is:

aura

2+ E, Ep

where it is assumed that the'contact potential 618 negligible. Equation (1) is plotted in Fig. 3 of the drawing, for

0 and 0.9-

and the change in bias required to give a certain balance (B) is given by:

Solving equation (1) for rmada By means of equation (3) it is possible to calculate the change in bias required for every value of voltage input to this circuit element to produce any desired load characteristic.

7 The arrangements of the invention, as shown in Fig. 1, might be utilized as a voltage limiting device in instances where it is desired that the output voltage increase linearly with the input voltage'up to a critical value-and remain constant for further in creases of the input voltage beyond that point. Under such conditions, the device might be so placed that transmission would normally take place directly therethrough. In other words, line sections L and L would be directly connected in the main transmission circuit and the control circuit L might be bridged through suitable amplifiers and rectifiers across line L in order to be effected by the incoming voltages. The hybrid coil 1 would normally be unbalanced to permit transmission therethrough. The grid bias on tube 2 would be'so adjusted that when the input voltage increased to a critical value the balance of the hybrid coil would be effected to approach a better balance and hence effect a limiting value on the transmission therethrough.

In Fig- 2, the arrangements of the invention are shown as embodied in a specific type of voltage limiting circuit arrangement. In this arrangement the transmission line comprises the input section L; which includes a band pass filter 6, the lower cut-01f of which is greater than a frequency corresponding to the rate at which it is desired to limit the output. The input signal is divided at the transformer 8 into two branch circuits 9 and J 10. Circuit 9 includes a delay circuit 11 and an amplifier 12 and is connected by transformer 13 to the filter 7 and the output section L5 of the transmission line. The branch circuit includes an attenuator 13, a delay circuit 14, the hybrid coil-control tube combination of this invention, and an amplifier l6 and is connected througha reversal 25 to the transformer 13 and the output line section I1 Due to the reversal 25 the currents from the two branch circuits are applied to the output circuit L in opposition. hormally'tlie hybrid coil 15 is balanced so no transmission takes place over the branch 10. When the hybrid coil 15 is totally unbalanced, the amplifier and attenuator in these two paths may be so adjusted that the voltage at the output from the second path is 1.2 times that from the firstpath. The total delay in these two paths should be identical so that the turnover in the second path puts the two sets of voltages at the output transformer 13 in phase opposition for all frequencies.

In order to supply the controlling voltage for determining the unbalance of hybrid coil 15 three amplifiers 19, 21 and 23 are connected across the input of the branch pat-h 10, the outputs of which amplifiers operate three linear rectifiers 20, 22 and 24. The output of these rectifiers is connected to the D. (J. amplifier 18, filter 17 and thence to the grid of the control tube 26 whereby the hybrid balance of the hybrid coil 15 may be controlled. Three rectifiers are provided be cause it is necessary to combine potentials from rectifiers of different characteristics in order to supply the required grid bias for the tube 26, as will now be explained.

The required grid biases, as determined by equation (2), above mentioned, as functions tube 26- Assuming balance at a given grid' voltage, the curve of thevariation in'applied gridpotential corresponding toa given load characteristic will also depend on Whether.

transmission is normally to take place through the pathlO, or notin other Words, it will depend on whether hybrid coil 15 is normally balanced or normally unbalanced.

In Fig. 4 the fullline curves are plotted'for the case where hybrid coil 15 is normally unbalanced, curves at and b respectively representing the potential variations when balance occurs at Ep and EA- .9.

The dottedline curves 0 and d are corresponding" curves for the case where hybrid coil 15 isnormally balanced. It is assumed for the'purpose of determiningthese curves that the gain, in tandem with the gain circuit element of the invention, is such as to produce a voltage ratio through it for the completely unbalanced condition in the hybrid coil of 1.0 for through transmission and12 for the balanced condition.

To produce such bias changes as are necessary to produce the load characteristics that have been indicated as desirable for voltage limiters requires rather special rectifier characteristics. e may specify a rectifier characteristic such that the current flowing in the rectifier is zero for all negative applied potentials, and may be expressedas:

for all positive values ofe. It then we place a'negative D. C. bias on the rectifier of value E andimpress a sinusoidal voltage of cos wt, the D. C. output of the rectifier becomes:

. 2: 1p =il e 6 cos i B1r=cos 1 E Equation t) is plotted .in Fig. 5, where all of the coefficients except a, are zerofor the linear rectifier, all but a zero for theparabolic rectifier, etc.

Letus assume that the'hybrid; coil. is-tb be. normally'balanced; at a v F ila p so that the required grid'bias onthe tube-'26 will be as indicated by the curve d of Fig. 4. It becomes necessary to design a combination of recti-fiers that will supply such a grid bias.

A comparison of curve (l with the curves of Fig. 5 indicates that the use of parabolic or cubic rectifiers is impractical and'that better results may be obtained by a combination of linear rectifiers. By investigation involving trial and e-rror methods, it was found that the required characteristic could be approximated by the use of three linear rectifiers, used as follows:

Relative E, 1.5 Eu 5.0 E,

This combination of rectifiers used'with the balanced transmission voltage limiter yields the load characteristic shown in .6 The manner in which this result Was arrived at may be understood from a consideration of the curves of Fig. 7 Here, the dotted line curve represents the desired variation of grid bias for the tube 26. Full line curves d d and dis represent the characteristics ofthe rectifiers No. 1, No. 2 and No. 3. The char-' acteristic of rectifier No. 1 is such thatno rectified potential appears in its output until the applied voltage-is equal to the desired cutoff. Rectifier N0. 2 produces no rectified output until the applied'voltage is 1.5 times the cut-ofi, and No. 3 produces no output until the applied voltage is 5 times the'output. If the rectified potential of curve 61 be superposed on the normal grid bias of the tube 26 produced by source E and said source produces a potential corresponding to (+3 OOS'B'Ir sin Bwd- (3 2 sin Bar) Bar) 15-11 sin 15111 mus- 5111 fir) sir cos 510+. 4

the curve d results. It will be seen that the characteristic of the rectifier is too steep as the dashed part of curve al rises rapidly above the desired curve (Z, the departure beill) fier'No; 1 in order that, it may supply an opposing potential. Its efiective characteristic is, therefore, represented by curve (Z which has a downward slope. By adding this curve to curve 6Z1, curve 6Z results. The latter crosses below the desired curve ol, as shown in full line, but again rises above it, as shown by the dashed part of curve d The departure begins at a ratio of input to cut-0fi corresponding to -5. Here the third rectifier comes into play. Its output must be reversed in sign with respect to that of rectifier No. 1 as it must reduce the slope of the dashed part of curve d The addition of the potential represented by curve (i thus brings the resultant curve to the position plotted at 03 in full line. The curves al (Z and 03 are given the difierent slopes indicated by using the outputs of the rectifiers in the relative proportions 0.9 for No. 1, 1.0 for No. 2, and 0.85 for No. 3, as given in the preceding table.- Obviously, the curve resulting from combining the outputs out these rectifiers in these proportions: closely approximates the desired curved of Fig. 7 The departure of the resultant curve from curve (Z in Fig. 7 causes the departure from the ideal fiat characteristic above cut-ofi, as shown in Fig. 6.

Distortion can only be produced in the circuit of Fig. 2 (if the grid bias applied to the control tube be direct current varying at a sufficiently slow rate), by variation of the plate voltage of the control tube by the desired signal in the hybrid coil circuit. If this circuit element be placed at such a point that the voltages so placed on the control tube plate are negligible with res 'ect to the effective plate voltage (E d- E this source of distortion is rendered as ineii'ective as may be desired.

As has been previously pointed out with respect to Fig. 2, there is connected across the input of branch path 10. three amplifiers, the outputs of which operate threelinear rectifiers 20, 22 and 24. Rather than adjusting the biases on these rectifiers (E0 E0 and E0 in the required ratios (1, 1.5 and 5), these biases can be made equal and the input signals reduced by means of the potentiometers P P and P associated with the amplifiers in the ratiosof 1, and This enables the use of identical rectifier circuits and in no way alters the operation of thedevice. The potentiometers P P and P have such resistances that the'voltage current characteristics of the rectifiers are linear.

Since the outputfrom the three rectifiers must be connected to a common circuit, the resistance network comprising resistances R R R R R and R and potentiometer P is provided. This network, when all of these resistances are equal, isv a balanced double Wheatstone bridge such that voltages in any one rectifier can not appear in any other rectifier circuitas will be clear from the schematic diagram of Fig. 2(a) By suitable adjustments of potentiometers P P and P the proper proportions of rectifier outputs appear across potentiometer P (0.9, 1.0, and 0.85) and by proper reversal of connections in this resistance network, these increments oi voltage have the proper signs. The voltage across potentiometer P is applied to the input of a D. C. amplifier 18, the output of which is connected to a low pass filter 17. The cut ofi frequency of this filter is determined by the maximum rate at which it is desired that limiting take place. The output from the filter isapplied to a. resistance in the grid circuit of the control tube 16 tovary the grid bias thereof and hence vary the degree of balance in the hybrid coil 15. The grid and plate batteries (E0. and E6 for the control tube have such values that ao i While the invention has been disclosed as embodied in certain specific arrangements which are deemed desirable, it is understood. that it iscapableof embodiment in many and other widely varied forms without departing from the spirit of the invention as defined by the appended claims.

l/Vhat is claimed is:

1. A transmission system including a double VVheatstone bridge comprising four arms forming one bridge, one arm of said bridge comprising four arms forming a second bridge, a source'connected to one pair of opposite neutral points of said first bridge, a second source connected to the remaining neutral points of said first bridge, a third source connected to opposite neutral points of said second bridge, and a work circuit so related to said double VVheatstone bridge as to receive transmission from each of said sources.

2. A transmission system comprising a work circuit, a plurality of sources of rectified current for said work circuit, and a network for relating said sources to said work circuit, said network having a plurality of separate sets of terminals each set of which is conjugate with respect to the other sets of "terminals, said plurality of sources being con-1 nected to said separate sets of terminals, whereby transmisslon may simultaneously take place from said sources to said work circuit without one source reacting on the others.

3. A transmission system comprising a work circuit, more than two sources of rectified current for said work circuit, and a network for relating said sources to said work circuit, said network having a plurality of separate sets of terminals each set of which is conjugate with respect to the other sets of terminals, said plurality of sources being connected to said separate sets of terminals,

whereby transmission may simultaneously take place from'all of said sources to said work circuit without interaction between said sources.

-4. A transmission system comprising a voltage operated translating device, a plurality of rectifiers for supplying rectified waves to said voltage operated device, and a network for relating said rectifiers to said translating device, said network having a plurality of separate sets of terminals each set I with respect to the other sets of terminals,

means to connect said supply sources to different sets of said terminals, whereby each source will be conjugate with respect to the other sources, and means to relate said work a p circuit to the bridge so'as'to receive transmis sion from all saldsources.

6. A transmission system comprising a voltage operated translating device, more than two rectifiers for supplying rectified waves to said translating device, a double Wheatstone bridge network having a plurality of sets of terminals each set of which is conjugate with respect to the other sets of terminals, means toconnect the outputs of said rectifiers to difierent sets of said ter- 4o minals, whereby the rectifiers will be so related that each will be conjugate with respect to the others, and means to relate said trans lating device to said bridge so as to receive rectified waves from all said'rectifiers.

7. A transmission circuit comprising two line sections, a hybrid coil interconnecting said line sections, a balancing element connee-ted to the net work side of saidhybrid coil, an impedancev controlling device connected to the line side of said hybrid coil, a control circuitfor said impedance controlling device, a plurality of rectifiers for rectitying waves applied to one of said line sections, control means associated with said rectifiers' to control the relationship between the input and output waves thereof, said control means being adjusted to produce predetermined diflerent relationships between the input and output waves of the different rectifi ers, connections to combine the outputs of said rectifiers in sald control circuit in any predetermined polarity relationship with rei spect to each other, and determiningmeans associated with said connections todetermine the relative proportions of the outputs of said rectifiers, whereby the relationships between the input and output waves of theseveral rectifiers and the relative proportions of the outputs of said rectifiersmay be so predetermined, and the outputs combined in such relative polarity as to control the balance of said hybrid coil in a predetermined manner. 7

8. A transmission circuit comprising two line sections, a hybrid coil interconnecting said line sections, a balancing element connected to the network side" of said hybrid coil, an impedance con-trolling device con-' nected to the line side of said hybrid c:oil,"a control circuit for said impedance controlling device, a plurality of rectifiersfor rectifying waves applied to one of said line sections, control means associatedwith said rectifiers to control the relationship between the input and output waves thereof, said control meansjbeing adjusted to produce predetermined different relationships between the input and output waves of lationship with-respect to each other, and I determining means associated with said connections to determine the relative propor tions of the outputs of saidrectifiers in said control circuit, whereby the relationships between the input and output waves of the several rectifiers and the relative proportions of the outputs may beso predetermined and the outputs combined in such polarity in said control circuit as to produce a potential therein bearing a relation to a. wave applied to said line section such that the potential varies directly with the applied wave for all values of applied wave on one side of a cutofi value while remaining substantially constant with changes of applied wave on the V other side of the cut-off value.

9. A line section divided into two branches, said branches having their output circuits connected in opposition toeach other to another section of said line, means including a vacuum tube for varying the transmission equivalent of one of said branches, a plurality of rectifiers associated with said line, control means associated with said rectifiersto control the relationship between the input and output waves thereof", said controlling means being adjusted to produce predetermined different relationships between the input and output waves of the different rectifiers, means to combine the outputs'of said rectifiers inany predetermined polarity re-' lationship with respect to each other, deter-- mining means, associated with said'connections todetermine the relative proportions of the outputs of said rectifiers, whereby the relationships between the input and output waves of the several rectifier's and the rela-' tive proportions of the outputs of said rectifiers maybe so predetermined and the outputs combined in such polarity as to apply a potential to said vacuum tube bearing a relation to a wave applied to said line such that the potential varies directly with the applied wave for all values of applied wave on one side of a cut-off value while remaining substantially constant with changes of applied wave on the other side of said cut-oil value. a s

i 10. A transmission circuit comprising two line sections, a hybrid coil interconnecting said line sections, a balancing resistance connected to the network side of said hybrid coil, the plate circuit of a vacuumtube connected to the line side of said hybrid coil, a control circuit for varying the impedance of said vacuum tube plate circuit, a plurality of translating devices having different characteristics and each translating waves from one of said line sections and applying the translated product to said control circuit, the translating devices being so chosen with respect to their input-output characteristics that their outputs may be combined in said control circuit to produce a controlling potential in said control circuitwhich varies with the wave applied to said line section in a predetermined manner for all values of applied wave on one side of a cut-off value and which remain substantially constant with changes of the applied wave on the other side of said cut-oil value. 'c

y 11. A transmission circuit comprising two line sections, a hybrid coil interconnecting said line sections, a control circuit for controlling the degree of unbalance in said hybrid coil, a plurality of translating devices having different characteristics and each translating waves from one of said line sections and applying the translated productv tosaid control circuit, the translating devices being so chosen with respect to'their input-output characteristics that the outputs .may be combined in said control circuit to control the degree of balance existing in said hybridcoil so that the coil willbe substantially balanced for all. values of applied voltage on one side of a cut-off value and will be unbalanced for all valueson the other side of'said cut-oil'- value.

12. A transmission circuit comprising two line sections connected in conjugate relationship with each other, a control circuit, means in said control circuit responsive to voltages impressed thereon for varying the degree of conjugacy in :the relationship between said line sections, a plurality of translating devices having different characteristics and each translating waves from one ofsaid line sections and applying the translated product to said control circuit, the translating devices being so chosen with respect to their inputoutput characteristics that the outputs may be combined in said control circuit to produce a voltage therein such that thevoltage re sponsive means in the control circuit will vary the degree of conjugacy in the relation--.

ship between the line sections to render the j section's substantially conjugate for all values of impressed voltage on one side of a cut-oil value while rendering them nonconjugate for all values on the other side of the cut-off value. 13. A line divided into two branches, said branches having their output circuits con-- nected to another section of the line 111 op.

position to each other, means in one of said branches controlled by the voltage applied thereto for regulating the gain in said branch, said means comprising a hybrid coil interconnecting two sections ofsaid branch circuit, a balancing resistance connected to the network side of said hybrid coil, the plate circuit of a vacuum tube connected to the line side of said hybrid coil, a control circuit-connected to one of thetwo sections of said branch circuit for varying the impedance of said vacuum tube circuit, a plurality of translating devices having different characteristics and each translatmg waves from one of said line sections and applying. the translated product to said control circuit, the translating devices being so chosen with respect to their input-output characteristics that their;

outputs may be combined in said control circuit to produce a controlling potential therein which varies with the appliedwave in a predetermined manner for all values of applied wave on one side of a cut-ofi value and which remains substantially constant with changes of the applied wave on the other side of said cut-off value.

14. A line divided into two branches, said branches having their output circuits connectedto another section of said line in opposition to each other, a hybrid coil interconnecting two of the sections of one of sa1d branch circuits, a control cir cui't for controlthe coil -will be substantially balanced for all values of applied voltage on one side of a; cut-oil value and will be unbalanced for all values on the other side of said cut-off value. 15. A line divided into two branches, said branches having their output circuits connected to another section of said line in ,op-c

position to each other, ahybrid coil interconnecting two of the sections of said branch circuit, a control circuit connected to one of the sections of said last mentioned branch circuit, a vacuumtube device-in said control circuit having its plate circuit connectedto the line side of said hybrid coil, a plurality of translating devices in'said control circuit, said translating devices having different characteristics and each translating Waves from one of said line sect-ions and applying the translated waves to said vacuum tube device, said translating devices being so chosen with respect to their input-output character istics that their outputs may be combined to supply the said vacuum tube device control ling potentials which vary with waves applied to said line in apredetermined manner for all Values of applied wave on one side of a cutoff value and which remains substan tially constant with changes of applied wave on the other side of said cut-oil value;

In testimony whereof, I have signed my name to this specification this 24th day of June, 1929.

WILLIAM T. WINTRINGHAM; 

